This invention relates to improvement to earth-boring tools, especially to steel-tooth bits that use hardfacing containing carbide particles to enhance wear resistance.
The earliest rolling cutter earth-boring bits had teeth machined integrally from steel, conically shaped, earth disintegrating cutters. These bits, commonly known as xe2x80x9csteel-toothxe2x80x9d or xe2x80x9cmill-toothxe2x80x9d bits, are typically used for penetrating relatively soft geological formations of the earth. The strength and fracture-toughness of steel teeth permits the effective use of relatively long teeth, which enables the aggressive gouging and scraping action that is advantageous for rapid penetration of soft formations with low compressive strengths.
However, it is rare that geological formations consist entirely of soft material with low compressive strength. Often, there are streaks of hard, abrasive materials that a steel-tooth bit should penetrate economically without damage to the bit. Although steel teeth possess good strength, abrasion resistance is inadequate to permit continued rapid penetration of hard or abrasive streaks.
Consequently, it has been common in the art since at least the early 1930s to provide a layer of wear resistant metallurgical material called xe2x80x9chardfacingxe2x80x9d over those portions of the teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast or macrocrystalline tungsten carbide dispersed in a steel, cobalt or nickel alloy binder or matrix. Such hardfacing materials are applied by heating with a torch a tube of the particles that welds to the surface to be hardfaced a homogeneous dispersion of hard particles in the matrix. After hardfacing, the cone is preferably heat treated, which typically includes carburizing and quenching from a high temperature to harden the cone. The particles are much harder than the matrix but more brittle. After hardening, the matrix has a hardness preferably in the range from 53 to 68 Rockwell C (RC). The mixture of hard particles with a softer but tougher steel matrix is a synergistic combination that produces a good hardfacing.
There have been a variety of different hardfacing materials and patterns, including special tooth configurations, to improve wear resistance or provide self sharpening. Generally, the hardfacing applied to the teeth of new bits is in a pre-application ratio range of 50 to 80 percent carbide particles, typically about 70 percent, in a metal matrix of iron, nickel, cobalt or their alloys. The thickness of the hardfacing deposit on new bits is usually about {fraction (1/16)} to xe2x85x9 inch over the flanks, end portions and top of the crest of the tooth. Portions of the hardfacing may be somewhat thicker. The thicker portions are generally where the flanks intersect the crest. These thicker portions may be up to double that of other areas.
U.S. Pat. No. 5,791,423 teaches a thicker portion at the corner that is an intersection of the leading flank with the outer end. The patent indicates extra thick portions can be formed by applying a second layer of hardfacing over a first layer before the initial layer cools to room or ambient temperature. The first and second layers of hardfacing are applied from the same hardfacing rod. U.S. Pat. No. 4,726,432 discloses hardfacing the leading flanks of teeth with a larger particle size hardfacing, and hardfacing the trailing flanks of teeth with a smaller particle size, more wear resistant hardfacing. The difference in hardfacing causes a self-sharpening effect. U.S. Pat. No. 5,492,186 teaches hardfacing on an inner end, crest and extending over onto the outer end with a first hardfacing. The remaining portion of the outer or gage end is hardfaced with another hardfacing that is more wear resistant.
The earth-boring bit of this invention has at least one hardfaced region that has a first layer of hardfacing of a first grade of carbide particles within a metal matrix. A second layer of hardfacing has an overlapped portion overlaid with at least a portion of the first layer. The second layer also has carbide particles within a metal matrix. However, the second layer is of a different grade than the first grade. The first grade of hardfacing is tougher, but has less wear resistance than the second layer. The second layer has more wear resistance than the first layer.
In the preferred embodiment, the second layer has a carbide particle density that is greater than the first layer. Density, as used herein, refers to the average total volume of carbide particles within a unit volume of matrix metal. Preferably, both layers are made up of a majority of sintered tungsten carbide particles. One manner in which to achieve different density is by using different particle sizes in the first and second types of hardfacing. In the preferred embodiment, the majority of particles of the first layer are larger in average size or volume than the majority of the particles of the second layer, resulting in a lesser density. The bit also has portions that contain a single layer of hardfacing. The single layers may be of the same hardfacing as the first layer or the second layer.